The present invention relates in general to remote control systems and, in particular, to a remote control system for controlling selected functions of a television receiver.
Remote control systems for television receivers normally comprise a battery-powered, hand-held transmitter which encodes and transmits selected keyboard information and a receiver which decodes the transmitted information and generates the necessary control signals for operating the selected functions of the television receiver. Most such systems employ an ultrasonic transmission feature wherein the keyboard information is transmitted as a suitably modulated sound wave or, alternatively, utilize a transmission system operable in the infrared region of the spectrum for transmitting data at the speed of light.
Ultrasonic remote control systems, although extensively used in the prior art, exhibit various inherent limitations which are especially undesirable when considered in connection with the operation of a television receiver. Initially, due to the relatively narrow width of the frequency band employed, only a limited number of controllable functions may be accommodated by the system. Also, ultrasonic systems normally operate rather poorly for rejecting ambient noise in the ultrasonic spectrum and are quite sensitive to room reflections for initiating undesired remote control operations. In an effort to overcome these problems, there has been a recent tendency toward the increased use of infrared transmission systems. Such systems typically accommodate an increased number of controllable functions and can be designed for exhibiting less sensitivity to ambient noise than the ultrasonic systems.
In order to distinguish between selected keyboard information as well as to provide a degree of immunity from ambient noise, the transmitted infrared signals are typically encoded in a precise manner using any of a number of well known techniques, most of these techniques necessitating the use of highly stable crystal oscillators or the like which must be properly adjusted to facilitate accurate measurements of time or frequency. Exemplary of these techniques are various known forms of pulse position modulation and pulse width modulation. U.S. Pat. No. 3,928,760 to Isoda discloses another technique in which a remote control light signal is amplitude modulated by a modulating frequency in the ultrasonic range, the modulating frequency subsequently being detected for controlling a selected function. In U.S. Pat. No. 3,906,366 to Minami et al, a remote control system is disclosed in which a transmitted remote control signal is characterized by a gradually variable duty cycle. The receiver is adapted for discriminating the received signal depending on its frequency for identifying the function to be controlled while the duty cycle is detected for allowing the controlled function to be continuously and gradually operated. U.S. Pat. No. 3,866,177 to Kawamata et al discloses another remote control system in which a high frequency remote control signal is chopped with a lower frequency signal. The receiver includes a plurality of tuned circuits each responsive to a different high frequency signal and means for integrating several cycles of the lower frequency signal for activating a selected function.
It will thus be appreciated that each of the above systems employs means for discriminating a transmitted remote control signal on the basis of frequency or some other relatively precise time relationship. As a result, tuned circuits, synchronized clock generators or other similar devices which frequently require adjustment and add to the cost and complexity of the system are normally required to enable the transmitted signals to be properly decoded. It is a primary object of the present invention to provide an infrared remote control system capable of operation over a wide range of system variables and not requiring the use of such circuits for detecting precise time and frequency relationships.